Terminal units, also referred to in the art as by-pass boxes, are known and are usually of rectangular shape. They have an inlet at one end and an outlet at another end and conditioned air normally flows straight through the box. A by-pass opening may be provided in a top or a side wall of the box, and a diverter element, such as a pivoted blade, is positioned within the box to deflect all or some of the conditioned air entering the inlet to the said by-pass outlet upstream of the outlet. The underlying principle of the air handling system in which by-pass boxes are used is that under all operating conditions, the pressure delivered by the supply fan not decrease and should ideally remain constant. This is due to the fact that in low cost air handling systems, the supply fans used are usually of the forward curve type. These fans have the inherent characteristic of compensating for a drop in system pressure by increasing the air volume at a rapid rate with the result that the power required to drive the fan increases. This then implies that the by-pass box must be able to cycle the supplied air between the space and the by-pass outlet without creating any changes to the air flow at the by-pass box inlet at all positions of the diverter blade throughout this cycle.
The underlying problem with existing by-pass units arises when the diverter blade or plate is at or near the halfway position. In duct systems, pressure is required at the entrance to the system to overcome frictional losses of the air as it moves through the duct system. The quantity of pressure required to move the desired amount of air through the ducts varies with the square of velocity of the air in the ducts. To reduce the quantity of air in a given duct system while maintaining the same pressure at the entrance, it is necessary to increase the resistance to the flow of air in the duct system, i.e., reduce duct size, add restrictions such as dampers, lengthen the duct, etc. For example, let us examine three duct systems: one from the supply fan to the inlet of the by-pass box, one from the inlet of the by-pass box to the conditioned space and one from the inlet of the by-pass box through the by-pass outlet and a manually adjustable balancing damper. Let us suppose for case of argument that through the use of the manually adjustable balancing damper the frictional losses are equal in the two latter duct systems in this example. As the diverter blade moves the conditioned air from one of these two duct systems to the other, the velocity in one duct will drop as the other will increase from zero. In the mid-cycle position of the diverter blade, the air velocity in each duct system will be half of its previous maximum: there are now two outlets for the same quantity of air. With the reduction in air velocity, the frictional losses have been reduced and less pressure is now required to deliver the air to the conditioned space and the by-pass outlet. In response to the change in pressure requirements, the velocity through all three duct systems will then increase until the loss of pressure has been compensated for. This translates into an increase of the airflow at the supply fan. In the case where several by-pass boxes in the system are subjected to the same conditions, the increase in airflow at the supply fan may become more than the supply fan motor can handle.
In known by-pass boxes on the market today, supply fan motors must be oversized to protect against the above eventuality and use more power than would be theoretically necessary if no variations in system pressure were created by the by-pass box. In prior art, several shapes and methods of mounting the diverter blade have been proposed with little or no success in overcoming this problem. In addition, they may create additional problems. In its simplest form, the diverter blade takes the form of a plain rectangular plate that is pivoted on one edge and mounted with the pivot axes close to the wall of the duct adjacent to the by-pass outlet opening. The blade travels between a position in which it closes the opening so that all of the air flows straight through the duct, and a position in which the blade blocks the outgoing air so that the air is totally diverted by the blade into the by-pass opening. Typically, an actuator is mounted externally on the duct and coupled to the shaft for turning the shaft between its two extreme positions under the control of a thermostat in the space to be conditioned. Since pressure is required to move the air from the by-pass box to the conditioned space, it exerts a force perpendicular to the blade that tends to rotate the blade to its extreme positions. The velocity of the air impinging against the blade also exerts an additional pressure and thus a force that tends to rotate the blade out of the air stream. Typically the forces associated with the pressure to overcome the system frictional losses is two to five times the pressure generated by the air velocity. Two problems may arise: 1. The actuator must resist these forces and must supply a relatively high torque to move the diverter blade at or near the extremes of its travel. Relatively powerful actuators must therefore be used with their adjacent additional cost. 2. When the pressure losses downstream from the unit are relatively high, the diverter blade will have a tendency to pulsate as it approaches the extremes of its travel: the pressure differential across the blades is at or near its maximum. In some cases, the diverter blade may flex and snap closed over the opening, causing an audible noise. Also, with the diverter blade at mid-cycle, the supply fan will sense a reduction in pressure and the related problem as described previously.
In an alternate attempt to address these problems, it has been proposed to relocate the pivot axe to the center of the rectangular blade and extend it perpendicular to the airstream through the center of opposite faces of the rectangular duct. In this configuration, part of the blade is above the pivot shaft and an equal part below so that the turning effect on the shaft imposed by the pressure and air velocity impinging on one part of the blade is counteracted and ideally balanced by the pressure and air velocity that impinges on the other part of the blade. While this blade arrangement avoids the imposition of high torque loads on the blade pivot shaft, an auxiliary blade must be provided to close the by-pass outlet in the duct when all of the air is to flow straight through. Normally, the auxiliary blade is pivoted through the center or less ideally to the edge of the by-pass opening and out of the main air flow through the duct, and the auxiliary blade is coupled to the main diverter blade by a linkage so that the auxiliary blade is opened and closed automatically in response to the turning of the main diverter blade under the control of the actuator. This arrangement not only introduces additional components and, therefore, cost and attendant service difficulties, but the problem of the pressure drop at mid-cycle is still present. In the case where the auxiliary blade is pivoted at its edge, the actuator must also overcome the additional load of the pressure acting against the blade.
In an attempt to eliminate the additional auxiliary blade, it has been proposed that the blade have an angled shape with an intermediate blade portion selected so that, in the diverting position, the blade presents to the incoming air, surface portions of substantially similar area disposed on respectively opposite sides of the shaft and, in the straightthrough position, a portion of the blade closes the bypass outlet. While this arrangement will ideally eliminate the forces attributed to the air velocity, the higher forces due to pressure acting perpendicular to the blade are not balanced since the surface portions of the blade as seen by this pressure are not equal on both sides of the pivot shaft. This proposal is also subject to the pulsation and flexing problems noted for the edge pivoted diverter blade and only slightly addresses the problem of the drop in pressure at mid-cycle.